The present invention relates to an optical probe for use in a two- or three-dimensional measuring apparatus for measuring a surface shape such as the shape of a free surface, e.g., an aspherical lens or the like, surface roughness, a step difference, etc., at low pressures and with super high accuracy.
In order to measure the shape of a free surface such as an aspherical lens or the surface roughness thereof, the currently-required measuring accuracy is on the order of a submicron to 10 nm, and therefore, conventional three-dimensional measuring apparatuses do not fit such highly accurate measurements. Meanwhile, Japanese Patent Application Nos. 57-189761 (189761/1982) (corresponding to U.S. Pat. No. 4,611,916) and 60-148715 (148715/1985) (corresponding to U.S. Pat. No. 4,776,699) disclosed a measuring apparatus utilizing an optical probe with the aim of measuring even an aspherical surface, a free surface, etc., with a sufficiently high measuring accuracy. The disclosed apparatus is designed to condense light onto the the surface to be measured to thereby to measure the shape of the surface from the reflecting light.
Since the above-described type of measuring apparatus makes measurement based on the light reflecting from the surface to be measured, however, it is impossible for the apparatus to measure the shape of a surface which is coated so as not to reflect light. Further, if the surface to be measured is inclined more than 30.degree., the surface cannot be measured from orthogonal coordinates.
A conventional contact probe of a three-dimensional measuring apparatus does not have a sufficiently large rigidity in the lateral direction and has large contact pressure. Therefore, if the surface to be measured is inclined, for instance, 45.degree., the same amount of contact pressure acts on the probe both in the Z-direction and the XY-direction, causing the lower end of the probe to shift in the lateral direction, which results in measuring errors not smaller than 1 .mu.m. Accordingly, a measuring accuracy not larger than 0.1 .mu.m cannot be attained by the conventional contact probe.
Similarly, since the measuring pressure is high and the radius of curvature of the lower end of a stylus of a conventional surface roughness measuring apparatus is small, the surface to be measured undesirably damaged or scratched. When the surface to be measured is greatly inclined, the apparatus generates measuring errors due to the lateral shift of the stylus.